1. Field of the Invention
The present invention relates to an image scanner, particularly a desk stand type image scanner that scans the face containing the image data to read of an original page placed with its face up.
2. Description of the Related Art
In conventional desk stand type image scanners, image reading means for reading the image data on an original placed on the original setting table consist of a one-dimensional image sensor, a beam condensing lens, and a reflecting mirror. In addition to the primary scan, a secondary scan of an original is performed by the rotation of the reflecting mirror, and thus two-dimensional images are obtained. Despite of whatever environments where such image scanners be installed, the antecedent machines have rotated the reflecting mirror to carry out the secondary scan at a constant speed to scan an original""s face containing the image data to read at a uniform rate.
Consequently, these machines have sometimes failed to obtain high quality images if placed in dark surroundings less than an allowable range of brightness, since there was not the light quantity enough to assure the operation of the one-dimensional sensor used in them, resulting in a poor signal-to-noise (S/N) ratio. In light surroundings more than an allowable range of brightness, conversely, the light quantity is too much for the one-dimensional image sensor operating speed. This also has led to such trouble that the output of the one-dimensional sensor is saturated, causing image quality degradation. Thus, the operation of conventional desk stand type image scanners have only been assured in a limited range of brightness in the-environments where they are installed.
In a prior art reference, Japanese Published Unexamined Patent Application No. Hei 9-233265, such a desk stand type image reader has been disclosed that features: an image input unit includes an imaging lens and a one-dimensional image sensor; a hood is provided which just covers the maximum area in which the image input unit can read; the hood shuts out disturbance light from the surroundings of the location where the image input unit is installed; and lighting means are provided inside the hood. This image reader is, however, inevitably large in size, because it has the above-mentioned hood and the lighting equipment inside the hood.
An object of the present invention is to provide an image scanner of concise structure, enabling the expansion of the range of brightness within which the scanner can operate, while maintaining the image quality obtained by the image reader.
An image scanner according to the invention, equipped with a one-dimensional image sensor and an optical mechanism to scan an original placed with its face up in a limited area and makes the sensor read the image data on that face, comprises brightness sensing means for sensing the ambient brightness and means for varying the reading speed of the one-dimensional image sensor in accordance with the brightness sensed by the brightness sensing means.
The means for varying the reading speed of the one-dimensional image sensor comprise clock generating means including an A/D converter for converting an analog value of brightness that is output from the brightness sensing means into a digital value, a latch for latching each converted digital value of brightness, a ROM for performing the table conversion of each latched brightness value, an oscillator which outputs a basic clock signal, and a counter which divides the frequency of each basic clock signal by a fraction based on the table-converted value of brightness at any given timing; as well as a driver motor with its rotation speed varying, according to a clock signal that is output from the clock generating means.
The optical mechanism to make the one-dimensional image sensor read the image data on an original page may be implemented in either of the following manners: scanning the original""s face containing the image data to read by focusing to a fixed sensor with a rotary mirror and fixed lens; and scanning the original""s face by the movement of the lens and the image sensor. In either case, the light reflected from the original""s face is allowed to strike on the one-dimensional image sensor.
The ambient brightness sensing means will output average illumination continuously or intermittently, depending on the illumination in the room where the scanner is placed.
Furthermore, the above brightness sensing means are realized by a brightness sensor which may be oriented either opposite to or directly toward the face of an original that contains the image data to read.